1. Field of the Invention
The present invention relates to a control circuit of a power converter for precisely controlling output voltage from a power converter which has a plurality of switching devices and which is capable of obtaining AC conversion output.
2. Description of the Related Art
FIG. 57 is a block connection diagram equivalent to a conventional inverter control circuit of a type, for example, disclosed in "Inverter Output Voltage Waveform Closed Loop Control Technique", p.205 to 212, a collection of theses, FIFTH INTERNATIONAL TELECOMMUNICATIONS ENERGY CONFERENCE (oct, 18 to 21, Tokyo), Intelec 83.
Referring to FIG. 57, reference numeral 1 represents a main inverter circuit, 2 and 3 respectively represent a reactor and a capacitor which form an AC filter, 4 represents a DC power source, 5 represents a load, 7 represents a drive circuit for the main inverter circuit 1, 801 represents an AC sine-wave reference oscillation circuit, 802 represents an amplitude command generating circuit and 803 represents a voltage control amplifier. Reference numeral 804 represents a pulse-width modulation circuit (hereinafter called a "PWM circuit") comprising, for example, a comparative circuit 804a and a carrier wave generating circuit 804b as shown in FIG. 58. Reference numeral 811 represents a multiplier and 812 represents an adder/subtracter.
The operation of the circuit shown in FIG. 57 will now be described.
A portion of the circuit between the terminals of the capacitor 3 thereof enables output voltage to be obtained, the level of which corresponds to control output PWMO from the PWM circuit 804. On the other hand, the output from the AC sine-wave reference oscillation circuit 801 and that from the amplitude command generating circuit 802 are multiplied in the multiplier 811 so that output voltage command V.sub.C * is made. Then, the deviation between the output voltage command V.sub.C * and output voltage V.sub.C is calculated by the adder/subtracter 812. The voltage control amplifier 80.3 and the PWM circuit 804 control the switching operation of the inverter 1 to make the foregoing voltage deviation to be zero.
Since the conventional apparatus for controlling the power converter, such as the inverter, has been constituted as described above, there arises the following problems.
(1) The main inverter circuit 1 is operated as a very low impedance voltage source when viewed from the output side thereof. Therefore, a problem of short circuit sometimes rises on the load side of the main inverter circuit 1. Another problem arises if an in-rush current flows due to a supply of transformer voltage in that an excess flow of the output electric current easily realizes an excess current state and therefore protection of circuit elements becomes difficult.
(2) Since the voltage control system of the main inverter circuit 1 includes an LC filter having an unsatisfactory damping characteristic, the voltage control amplifier 803 cannot easily be so designed as to prevent the resonance of the foregoing filter to stabilize the voltage control system.
(3) Since the reactor 2 is, in series, connected to the output terminal of the main inverter circuit 1 and the voltage drop of the reactor 2 is changed to follow the electric current flowing through the load 5, there arises a necessity for the voltage control amplifier 803 to completely and quickly compensate the foregoing voltage drop in order to control the output voltage to be a desired value. Therefore, even if no precise output voltage is needed, the output voltage control performed by the voltage control amplifier 803 is needed to compensate the voltage drop in the reactor 2. If precise output voltage is needed, the design must be so made that the voltage control amplifier 803 has a large gain. However, the stability of the voltage control system limits the gain enlargement, and therefore overshoot or undershoot of the output voltage corresponding to the response of the voltage control system undesirably takes place if the load has been changed rapidly.